Help with DDos Type UDP Flood

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I have a CentOS 5.3 server hosting our companies website. The connection to the site is through a Cisco RVS4000 security router. The website is the only device on the network that uses this router and the router only has one inbound rule which forwards port 80 to the website.
I setup the Linux environment following instruction I found here for CentOS 5.3 over a year ago. Lately, we have been getting hit with various DoS attacks. Being a Linux novice, I'm looking for some help in preventing these DDoS attacks.
When the most recent attack occurred, I actually see traffic logged coming fro
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